A major problem of neuroscience is to understand mechanisms for behavioral recovery following neural injury. Our collaborative work supported by this grant has provided the first demonstration that capacity for non-associative learning is restored by regeneration of neuronal connections, that a specific connection is crucial for sensitization, although return of function does not immediately follow reconnection, and it suggests new mechanisms for non-associative learning. It is the basis for the proposed cellular and behavioral studies on loss and recovery of the capacity for sensitization following damage to central circuits. Recent work indicates that vertebrates and invertebrates share fundamental mechanisms for axon growth and guidance in addition to neuronal function. In leeches we combine in-depth work on modulation of defensive shortening from one laboratory and work on synapse regeneration by sensory neurons and interneurons including the S-cell from the other. Thus we have shown that the capacity for sensitization of reflexive shortening requires the S-cell, which is restored by regeneration of one of the S-cell's synapses, but with a delay. Proposed experiments will give key information about cellular changes that underlie learning as related to mechanisms for nervous system recovery from injury. Advantages of leeches for such studies include (1) their identifiable neurons, able to re-establish specific connections, (2) selective laser-cutting of single axons in living animals, and (3) stable recordings from identified neurons in behaving animals. Experiments will determine (1) mechanisms for axotomy-induced loss of capacity for sensitization, (2) mechanisms that restore the full capacity for sensitization after injured S-cells reconnect, and (3) the relationship between regulation of functional geometry and repair of intersegmental sensory projections. Methods will include electrophysiology, injection of intracellular markers, laser microbeam axotomy, behavioral testing of leeches including semi-intact preparations, histochemistry, immunocytochemistry, immunoblotting, and confocal and electron microscopy. The studies will reveal basic mechanisms for restoration of plastic properties of the nervous system after injury while at the same time provide insights into the cellular circuitry for non-associative learning.